solid state drivesmeseec.ce.rit.edu/551-projects/fall2011/1-3.pdfarchitecture x8 x8 / x16 endurance...

Solid State Drives

By:Kevin AlderferMichael Bova

Thursday, November 10, 11

Overview

• SSD Basics

• HDD vs. SSD

• History of the SSD

• Flash Memory• NOR Type• NAND Type

• SLC• MLC

• Problems

• Today’s Solutions

• The Future of Data Storage


The Basics

• No Moving Parts

• Data stored in non-volatile flash memory chips

• Charge carriers are stored completely within the solid material

• No magnetism involved


HDD vs. SSDThursday, November 10, 11

HDD vs. SSD SpecsAttribute SSD HDD

Defragmentation No benefit Often required

Sound No sound Moving parts will make noise

Mechanical Breakdowns No moving parts All moving parts have a chance of

failure

Environment Not impacted by shock, altitude, or vibration Susceptible to these factors

Magnetism No impact Magnetic surges can alter saved data

Parallel Operation

Some controllers can have multiple chips reading and writing

different data simultaneously

Have multiple heads but are all connected. Can only write one thing at

a time.

Write Longevity

Limited writes (1-5 million). Software controllers manage this and can help SSDs last decades.

DRAM SSDs aren’t limited.

Unlimited writes, but eventual mechanical failure


HDD vs. SSD Specs (cont.)

Attribute SSD HDDRandom AccessTime

.1ms 5-10msmove heads and rotation

Read Speed ~250 MB/s ~100 MB/s

Write Speed ~250 MB/s ~60 MB/s

Encryption Must erase data before overwriting Can directly overwrite data

Cost NAND SSDs ~ $.90-2.00 per GB ~ $.05/GB for 3.5” & $.10/GB for 2.5”

Storage Current Max: 2TBTypical: 64-256GB

Current Max: 2-3TBTypical: 500GB-1TB

PowerFlash based: 1/2 - 1/3 power of

HDDDRAM based: same as HDD

High performance: 12-18 wattsLaptop drives: 2 watts


History of the SSD• Intel 1978 - Electrically Erasable Programmable Read Only Memory (EEPROM)

• Floating-gate transistors to store bits• About performance of RAM without volatility• Full disk system about speed of hard disk• Not much faster than HDD, but consumed 100 times the power• Much more expensive than HDD

• Toshiba 1980 - NOR Flash Memory

• Used much less power than EEPROM

• Allowed for more read/write cycles before burn-out

• 1989 - NAND flash

• Densely packed “pages”

• Much cheaper than NOR

• Reads have a much higher bit error rate, however

• MLC NAND flash

• Each cell of MLC has at least four states (whereas SLC has two) capable of storing at least two bits.

• This requires much finer, more precise measurement, and therefore results in a higher probability of error.

• Cheaper than SLC, but slower and more error-prone. As technology increases, more bits can be stored by MLC


Flash Memory

A large Vg bumps up electrons onto Floating Gate from inversion layer

Electrons closer to Drain have more momentumEnough energy can bump them into Silicon atoms

The number of electrons on the Floating Gate affects Vt

Vt measured to determine state of cell

Limited lifespan due to Si degradationThursday, November 10, 11

NOR Characteristics

• Intel- 1980

• Traditionally used in portable electronics

• Cells connected in parallel, allowing for random access

• Low density with high read speeds

• Slow write: Blocks must be written with zeros before they can be erased and rewritten.

• Good for code execution (XIP- eXecute In Place)


NAND Characteristics• Toshiba- 1989

• Very small cell size at the cost of cell parallelism (Resulting in indirect access)

• Eight memory transistors connected in series

• Replacing NOR due to faster write/erase, higher density, and lower cost-per-bit (less expensive)

• Write/erase whole blocks at a time: Much faster than NOR.

• Good for storage purposes

• Consumes less power

• Recent Density: 32GB on MicroSD card the size of a fingernail


NAND vs. NOR: A ComparisonThursday, November 10, 11

Single-Level Cell

0 - Programmed

1 - Erased

Represents 1 bit

High performance and long term reliability


Multi-level Cell

Generally Represents 2 bits

Must have rigidly controlled programmingPrecise amounts of charge storedPrecise Vt reading


SLC vs MLC Specs

SLC MLCMLC

Density 16Mbit 32Mbit 64Mbit

Read Speed 100ns 120ns 150ns

Block Size 64Kbyte 128Kbyte128Kbyte

Architecture x8 x8 / x16x8 / x16

Endurance 100,000 cycles 10,000 cycles10,000 cycles

Operating Temperature Industrial

Commercial Commercial


SLC vs. MLC Overview

SLC MLC

High Density

Low Cost per Bit

Endurance

Operating Temperature Range

Low Power Consumption

Write/Erase Speeds

Write/Erase Endurance


Problems

• Lifetime• Algorithms today keep track of block’s write count, and remaps the logic sectors of

block about to fail to new blocks.

• With one extra GB of surplus NAND flash, a single logical block could be written every second for several years without failing.

• Therefore, it is now very difficult to wear out an SSD unless you are very committed to doing so.

• Random-Write-Hole (RWH)• I/O Operations/sec slows from 1000s to 10s

• Heavily-polled dies burdened down by a gridlock of huge writes from small write requests, because it’s desirable to write full blocks

• Read Modify Write cycle (RMW)• SSD will perform RMW cycle when writing to the location of a formerly “deleted” file

• This adds an extra cycle every write once the drive is filled

• Taken care of by TRIM


TRIM• Definition: A software command that allows an

Operating System to inform a SSD which blocks of data are invalid due to user or OS generated erases

• Enables SSD to handle garbage collection and free up space

• Necessary since SSDs operate vastly differently than HDDs at a low level


RWH Fixes

• Sandisk: nCache• Software write buffer• Small section on disk (320MB for 64GB SSD)• Queues up small write and cleared out during idle moment• May not have idle moment for a long time, so won’t allow sustained

high levels of writing

• Intel: Sector Remap• Pools a number of small writes, in multiples of page size, on one block• Rewrites sector map table to combine these pages into a logical sector• X25-M, in G1 and G2 models, has the best RWH solution today

• G1 and G2 models have new firmware to fix fragmentation issue in earlier models


Into the Future• SSDs will continue to improve in lifespan and cost.

• Holographic disks (Being developed by GE) utilize multiple layers of solid material to store exponentially more data than current single layer disks.

• Molecular memory: Using organic molecules instead of silicon may become a possibility in the future. The size of such molecules is in the order of trillions of times smaller than the silicon transistors used today.

• Bacteria: Bio-genetically encoded DNA.

• Quantum Computing: Storing data in the properties of quantum mechanical systems (i.e. an electron’s spin)


THE END


solid state drivesmeseec.ce.rit.edu/551-projects/fall2011/1-3.pdfarchitecture x8 x8 / x16 endurance...

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